Optical semiconductor device with multiple quantum well structure

ABSTRACT

An optical semiconductor device with a multiple quantum well structure, in which well layers and barrier layers comprising various types of semiconductor layers are alternately layered, in which device well layers ( 6   a ) of a first composition based on a nitride semiconductor material with a first electron energy and barrier layers ( 6   b ) of a second composition of a nitride semiconductor material with electron energy which is higher in comparison with the first electron energy are provided, followed, seen in the direction of growth, by a radiation-active quantum well layer ( 6   c ), for which the essentially non-radiating well layers ( 6   a ) and the barrier layers ( 6   b ) arranged in front form a superlattice.

CROSS REFERENCE TO RELATED APPLICATIONS

Pursuant to 35 U.S.C. § 120, this application continued claims thebenefit of prior U.S. application Ser. No. 09/913,394, filed Feb. 5,2002 U.S. Pat. No. 6,849,881 which is a National Phase Application forInternational Application No. PCT/DE00/04089, filed Nov. 20, 2000, whichclaims priority to German patent application 19955747.0, filed Nov. 19,1999.

FIELD OF THE INVENTION

The invention relates to an optical semiconductor device with a multiplequantum well structure, in which well layers and barrier layerscomprising various types of semiconductor layers are alternatelylayered.

BACKGROUND OF THE INVENTION

A device of this type is known for example from EP 0 666 624 B1 or fromJournal of Crystal Growth 189/190 (1998) pages 786–789.

The high quantum efficiency of indium-gallium-nitride (InGaN)-based LEDsand laser diodes is caused by the self-organized growth of indium-richislands in the active quantum well. As a result, the injected chargecarriers are spatially localized at these islands and are prevented fromnon-radiating recombination at lattice defects.

The nucleation of these quantum dots must be initiated by suitablebuffer layers. In particular, indium-containing structures are suitablebefore the actual active zone as a buffer layer. Indium-containingnitridic semiconductors (Ga_(x)Al_(y)In_(1−(x+y))N semiconductors) havea tendency toward segregation and formation of indium-containing phases.This leads to varying strain fields at the growth surface, promoting theformation of indium-rich islands in the active quantum well. GaInNlayers approximately 100 nm thick can be deposited before the activezone in order to improve the GaInN quantum dot nucleation.

Previously, an optimum efficiency could be achieved with, for example,2- to 10-fold quantum well structures. As can be shown experimentally,the emitted photons are generated exclusively in the two uppermostquantum wells (i.e. those closest to the p side). A suitable choice ofgrowth parameters achieves the effect that the emission takes placeexclusively in the uppermost of the quantum wells. The lower quantumwells serve for improving the nucleation of the GaInN islands in theuppermost quantum well. Dispensing with them causes the optical outputpower to be reduced by over 50%. However, these quantum wells lead to aconsiderable increase in the forward voltage. The forward voltage can beimproved by reducing the number of wells at the expense of the quantumefficiency. The piezo fields, which lead to the observed increase in theforward voltage, can be compensated by high doping levels in the quantumwell region. However, this adversely effects the crystal quality of theactive layer or impairs the injection behavior and consequently reducesthe internal quantum efficiency.

BRIEF SUMMARY OF THE INVENTION

The invention is based on the object of improving in this respect anoptical semiconductor device of the type stated at the beginning.

The invention provides a multiple quantum well structure with welllayers and barrier layers comprising various types of semiconductorlayers which are layered alternately one on top of the other, in whichthe well layers are thin layers of a first composition based on anitride semiconductor material with a first electron energy and thebarrier layers are layers which are thicker in comparison, of a secondcomposition of a nitride semiconductor material with electron energywhich is higher in comparison with the first electron energy. Acting asthe radiation-active quantum well, seen in the direction of growth, isfirstly one of the last quantum well layers or the last quantum welllayer. The well layers arranged in front, which essentially do notradiate, and the barrier layers form a superlattice for this last layer.

In a particularly preferred development, in the superlattice, the welllayers are thin aluminum-gallium-indium-nitride layers and the barrierlayers are gallium-nitride layers which are thicker in comparison andthe active quantum well has indium-gallium-nitride.

Within at least one well layer of the superlattice, the In contentpreferably increases in the direction of growth, i.e. in the directionof the radiation-active quantum well layer. At the same time, the indiumcontent remote from the radiation-active quantum well layer ispreferably below 5%.

In a particularly preferred development, at least one of the well layersof the superlattice has at least one pair of single layers, of which thefirst single layer in the direction of growth has a lower indium contentthan the second single layer in the direction of growth. This well layerpreferably has a plurality of single layers whose indium contentincreases step by step from the single layer lying furthest away fromthe radiation-active quantum well layer to the single layer lyingclosest to the radiation-active quantum well layer. It is particularlypreferred for the steps of the increase in indium content to be smallerthan 5%. It is also particularly preferred for the indium content of thesingle layer furthest away from the radiation-active quantum well layerto be less than 5%. The thickness of the single layers preferably liesin the range of just a few monolayers, particularly preferablyapproximately one monolayer.

The particular advantage of the step-by-step increase in the In contentis that a potential profile resembling a delta potential is produced, inparticular in the case where the thickness of the single layers does notsignificantly exceed that of a monolayer. The difference in the energylevel in the barrier layers and the energy level obtained for oneelectron in the well layer is consequently not greater than in the caseof a rectangular well layer with a significantly lower indium content asthe uppermost single layer of the stepped well layer. This achieves theeffect that the advantages of a reduced overall indium content areretained, but the strain is at the same time influenced by the highindium content of the last single layer in such a way that thenucleation of InGaN-rich phases is improved and, consequently, thequantum efficiency is increased.

A further advantage arises for the following reason: the epitaxialgrowth of indium-containing III-V semiconductors (for example InGaAs,InGaN) is made more difficult by the relatively great atom radius of theindium. With constant fluxes of the group III components, indium istherefore incorporated with a delay. The indium concentration increasesduring the growth and approaches an equilibrium value exponentially.During the enriching phase, a certain indium coverage is formed on thegrowth surface. Only when this coverage is achieved is a constant amountof indium incorporated into the crystal. If, however, too much indium isoffered, the indium present on the surface forms metallic droplets,which reduce the quality of the deposited layers. In particular,however, such indium droplets produce shunts at the p-n junction,reducing not only the quantum efficiency but also the stability withrespect to electrostatic discharging (ESD). These effects are reduced byinitially depositing indium-containing layers with a low indium content,preferably less than 5%, then increasing the indium content insubsequent layers. In the case of the active layer itself, thesubstructure is then already prepared by the well layers in such a waythat InGaN compositions with a higher indium content can be deposited.

The nucleation of indium-rich phases is advantageously improved by theincreasing indium content in the well layer or layers. The harmfulformation of indium droplets is at least considerably reduced.

Further advantageous developments and refinements of the invention arethe subject of subclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is explained in more detail below on the basis of anexemplary embodiment with the aid of the figures, in which:

FIG. 1 a, b shows a schematic representation of the layer structure of adevice according to the invention,

FIG. 2 shows a schematic representation of the quantum well structure ofthe device according to FIG. 1,

FIG. 3 shows a schematic representation of the quantum well structure ofa customary type, and

FIG. 4 shows a schematic representation of the profile of the conductionband according to a particularly preferred embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

According to FIG. 1 a, initially a buffer layer 3 ofaluminum-gallium-nitride (AlGaN) is formed on a substrate 1 of siliconcarbide (SiC), to which a first contact electrode 2 is connected. Thisis followed by a further layer 4 of aluminum-gallium-nitride. A furtherbuffer layer 5 of silicon-doped gallium nitride is arranged over thelayer 4 and the quantum well structure 6 a, b, still to be explained inmore detail, is arranged over that, followed by the actual active layer6 c. Arranged over the active layer 6 is a further layer 7 ofaluminum-gallium-nitride, which serves as an electron barrier. Thislayer 7 is preferably doped with magnesium. A further GaN layer, notdesignated, may be arranged between the layers 6 and 7. Agallium-nitride layer 8, on which a second contact electrode 9 of thedevice is provided, is arranged over the layer 7.

The right-hand side of the layer structure shows in a schematicallyindicated manner the band gap of the single layers between the valenceband and the conduction band.

The buffer layer 3 functionally serves as a growth layer, which isrequired to allow the LED structure to grow on the silicon carbidesubstrate 1. The further aluminum-gallium-nitride layer 4 between thelayers 3 and 5 has an aluminum content which changes in the direction ofthe gallium-nitride layer 5. The gallium-nitride layer 5 is preferablysilicon-doped. The layer 7 of magnesium-doped aluminum-gallium-nitridearranged over the active layer 6 serves as an electron barrier.

This basic structure of FIG. 1 a can be used as standard forgallium-aluminum-indium-nitride light-emitting diodes (LEDs).

FIG. 1 b is an enlarged representation of the active layer 6 accordingto the invention. The layer with the quantum well structure 6 isconstructed by layers 6 a of gallium-indium-nitride (GaInN) beingarranged between individual gallium-nitride (GaN) layers 6 b. Theactually active, i.e. light-emitting, layer 6 c ofgallium-indium-nitride (GaInN) is followed by the uppermostgallium-nitride layer 6 b.

As can be seen, layers 6 a and 6 b of different thickness alternate. Thethinner layers 6 a of indium-gallium-nitride and the thicker layers 6 bof gallium-nitride in this case form superlattices, in which the wells 6a are thin, i.e. thinner than 3 nm, and the layers 6 b are 3 nm andabove. The layers are produced by vapor-phase or molecular-beam epitaxy.In this case, a slow growth of 1–10 nm/min, preferably 1–2 nm/min, atlow temperatures around 700° C. is intended.

The indium content lies below 24%, preferably however below 20%, and istherefore preferably reduced in comparison with customary indiumcontents. The layers 6 a and 6 b, depicted only once in the figure, maybe arranged repeatedly one above the other, the structure preferablybeing repeated x=3 times. The uppermost gallium-nitride layer 6 b isfollowed by the actually active, i.e. illuminating, layer 6 c ofindium-gallium-nitride.

It may preferably be envisaged to dope the quantum well structure 6 a, 6b with silicon in a concentration of 10¹⁷ to 10¹⁸ cm⁻³. This once againprovides a significant improvement in comparison with an undopedstructure.

FIG. 2 shows the energy conditions for the valence band VB and theconduction band CB. The electron energy is plotted in the direction ofthe y-axis, the quantum wells with a width corresponding to the layerthickness are plotted in the direction of the x-axis. The uppermostgallium-nitride layer 6 b is followed by the actually active layer 6 c.

FIG. 3 shows by comparison the valence band with thicker quantum wellsof gallium-indium-nitride than in the case of the invention. The effectof the piezoelectric fields produced by the strains is indicated by thesloping lines.

The use according to the invention of GaInN/GaN superlattices with thinwells (to a quantum well width of approximately 2 nm) in a layerstructure according to FIG. 1 and with quantum well conditions accordingto FIG. 2 allows the forward voltage to be significantly lowered and, atthe same time, the high internal quantum efficiency of theindium-gallium-nitride-based optical semiconductor device to bemaintained. The piezo fields otherwise forming are avoided entirely orhave virtually no effect any longer. In comparison with customary singlequantum-well structures, in which no gallium-indium-nitride superlatticeis deposited before the active well, the device structure according tothe invention has twice the conversion efficiency.

Superlattices are understood as meaning generally a particularlyperiodic sequence of layers only a few atomic layers thick. Thesuperlattice is separated from the active well by a GaN or AlGaN barrier(>3 nm). The silicon doping of the quantum well structure issignificantly improved in comparison with the undoped structure.

In comparison with known superlattice structures, the device structureaccording to the invention has a forward voltage that is lowered by >0.5V.

In comparison with SQW (Single Quantum Well) structures, in which noGaInN superlattice is deposited before its active quantum well, it hasbeen possible to double the conversion efficiency.

The combination of thin, and low-indium-content, optically inactivequantum wells (“pre-wells”) with an active quantum well 6 c allows theemission behavior of the previously known multiple quantum wellstructures to be maintained and the forward voltage to be lowered. Thethin GaInN quantum wells improve the quality of the active quantum well,while the low layer thickness of the “pre-wells” and their low indiumcontent have the effect of reducing the formation of disturbing piezofields. The forward voltage is therefore not increased by thisnucleation layer in comparison with a SQW structure.

FIG. 4 shows the profile of the conduction band in an InGaN well layer 6a between two GaN barrier layers, the GaN barrier layer 6 a comprising atotal of 4 single layers 60 a to 63 a. The indium content increases stepby step from the single layer 60 a lying furthest away from theradiation-active quantum well layer 6 c to the single layer 63 a lyingclosest to the radiation-active quantum well layer 6 c. The steps of theincrease in the indium content are smaller than 5% and the indiumcontent of the single layer 60 a lying furthest away from theradiation-active quantum well layer 6 c is less than 5%. The layerthickness of each of the single layers 60 a to 63 a lies in the range ofjust a few monolayers or corresponds approximately to one monolayer ofthe composition.

This produces a potential profile which is similar to a delta potential.Consequently, the difference between the energy level in the barrierlayers and the energy level obtained for an electron in the stepped welllayer is no greater than in the case of a rectangular well layer(represented on the right-hand side in the figure) with a much lowerindium content than the uppermost single layer of the stepped welllayer. This achieves the effect that the advantages of a reduced overallindium content are maintained, but at the same time the strain isinfluenced by the high indium content of the last single layer in such away that the nucleation of InGaN-rich phases is improved, andconsequently the quantum efficiency is increased.

The description of the invention on the basis of the above exemplaryembodiment is of course not to be understood as a restriction of theinvention to this embodiment. Rather, the invention also relates todevices in other material systems in which similar problems are to besolved.

1. An optical semiconductor device comprising a multiple quantum wellstructure the multiple quantum well structure comprises: at least onecombination of alternating essentially non-radiating well layers andbarrier layers, both comprising various semiconductor layers based on anitride semiconductor material; and a radiation-active quantum welllayer, disposed over the essentially non-radiating well layers andbarrier layers.
 2. The optical semiconductor device according to claim1, wherein the well layers comprise thin aluminum-indium-gallium-nitridelayers and the barrier layers comprise gallium-nitride oraluminum-gallium-nitride layers which are thicker than the well layersand the radiation-active quantum well comprises anindium-gallium-nitride layer.
 3. The optical semiconductor deviceaccording to claim 2, wherein the well layers and barrier layers aredoped with silicon.
 4. The optical semiconductor device according toclaim 3, wherein the dopant concentration is from 10¹⁷ to 10¹⁸ cm⁻³. 5.The optical semiconductor device according to claim 1, wherein theradiation-active quantum well follows an uppermost barrier layer.
 6. Theoptical semiconductor device according to claim 1, wherein layerthickness of the radiation-active quantum well is greater than layerthickness of the essentially non-radiating well layers.
 7. The opticalsemiconductor device according to claim 1, wherein the essentiallynon-radiating well layers are thinner than 2 nm and the barrier layersare at least 3 nm thick.
 8. The optical semiconductor device accordingto claim 1, wherein within at least one of the essentially non-radiatingwell layers and indium content increases in a direction of growth. 9.The optical semiconductor device according to claim 8, wherein in theessentially non-radiating well layers the indium content, remote fromthe quantum well layer, lies below 5%.
 10. An optical semiconductordevice comprising with a multiple quantum well structure the multiplequantum well structure comprises: at least one combination ofalternating well layers and baffler layers, both comprising varioussemiconductor layers based on a nitride semiconductor material, whereinat least one of the well layers is configured to produce a steppedpotential profile.
 11. The optical semiconductor device according toclaim 10, wherein the at least one of the well layers has at least onepair of single layers, of which a first of the at least one pair, in adirection of growth, has a lower indium content than a second of the atleast one pair in a direction of growth.
 12. The optical semiconductordevice according to claim 11, wherein the second of the at least onepair has an increased indium content of less than 5% than the first ofthe at least one pair.
 13. The optical semiconductor device according toclaim 11, wherein the indium content of the first of the at least onepair of single layers is less than 5%.
 14. The optical semiconductordevice according to claim 11, wherein the at least one well layercomprises a plurality of single layers whose indium content increases indirection of growth.
 15. The optical semiconductor device according toclaim 14, wherein the indium content increase is smaller than 5%. 16.The optical semiconductor device according to claim 14, wherein thethickness of each of the plurality of single layers comprises at leastone monolayer.
 17. The optical semiconductor device according to claim10, wherein the well layers are essentially non-radiating well layers.18. The optical semiconductor device according to claim 10, wherein aradiation-active quantum well layer is disposed over the well layers andbarrier layers.
 19. The optical semiconductor device according to claim10, wherein the stepped potential profile is similar to a deltapotential.